1. Field of the Invention
This invention relates to a heat-developable color photosensitive material and more particularly, to a low-cost heat-developable color photosensitive material providing a color image having a high image density and clear white areas.
2. Prior Art
Heat developable photosensitive materials and their image forming processes are well known in the art and described in the literature, inter alia, "Fundamentals of Photographic Engineering--Non-Silver Salt Photography--", Corona Publishing K.K., Tokyo, Japan (1982), pages 242-255.
A number of proposals have been made foe forming color images through heat development. For example, U.S. Pat. No. 3,531,286, 3,761,270 and 4,021,240, Belgian Patent No. 802519, and Research Disclosure, September 1979, pages 31-32 propose processes for forming color images through coupling of developing agent oxidants with couplers.
These heat-developable color photosensitive materials for forming color images, however, suffer from the serious problem that white areas are gradually colored upon exposure to intense light or long-term storage since the materials are of the non-fixation type and have silver halide left even after image formation. Further the above-mentioned processes require a relatively long time for development to complete and result in less desirable images having substantial fog and a low image density.
To overcome these drawbacks, U.S. Pat. No. 4,500,626, 4,483,914, 4,503,137 and 4,559,920 and Japanese Patent Application Kokai (JP-A) No. 165054/1984 propose to form or release a diffusible dye imagewise by heating and to transfer the diffusible dye to a mordanted image-receiving material with the aid of water or other suitable solvents.
This process still required relatively high development temperatures and the photosensitive material was not satisfactory in aging stability. In this regard, it was proposed to carry out heat development in the presence of a base or base precursor and a minor amount of water, followed by dye transfer for development promotion, development temperature lowering and simpler processing purposes (see JP-A 218443/1984 and 238056/1986 and EP 210,660 A2).
A variety of proposals have been made in the art for producing positive color images through heat development. For example, U.S. Pat. No. 4,559,290 proposes a method for forming an image by converting a dye providing (DRR) compound into an oxidized form having no dye releasing ability, preparing a heat-developable material in which the oxidized DRR compound is co-present with a reducing agent, carrying out heat development to oxidize the reducing agent in an amount corresponding to the exposure of silver halide, and allowing the remainder of the reducing agent unoxidized to reduce the oxidized DRR compound into the DRR compound to release a diffusible dye. EP 220,746 A2 and Japan Invention Society's Kokai Giho (Technical Report) No. 87-6199 (Vol. 12, No. 22) describe a heat-developable color photosensitive material including a compound capable of releasing a diffusible dye through a similar mechanism, that is, a compound capable of releasing a diffusible dye through reductive cleavage of a N-X bond wherein X is an oxygen atom, nitrogen atom or sulfur atom.
In heat-developable color photosensitive materials as mentioned above, a variety of polymers can be used as a binder. However, many polymers provide images with a low image density (Dmax) or high fog density (Dmin). It is difficult to find a compromise polymer between cost and image quality.
One known preferred class of polymer is gelatin. However, heat-developable color photosensitive materials using delimed gelatin are unsatisfactory in both Dmin and Dmax.